1. Field of the Invention
This invention relates to a pneumatically operable drive unit with an energy storage mechanism for shifting an associated actuator into a safety position when the auxiliary energy supplying the driving force fails.
2. Description of Related Art
Pneumatic drive units of this type serve to produce lifting and turning movements of actuators for throttling flowing media; see "Messen, Steuern und Regeln in der Chemischen Technik," Measurement, Control, and Regulation in Chemical Engineering!, 3rd edition, Volume III, Springer-Verlag 1981, p. 190.
Such pneumatic drive units are characterized, in particular, by their simple, sturdy designs and by the high actuating speeds that they can reach. As a rule, pneumatic drive units are designed to be single-acting so that the pneumatic force can act in only one direction. The required restoring force is usually produced by compression springs provided in the drive unit. These springs guarantee the movement of the actuator into the above-mentioned safety position if the pneumatic system fails.
It is known that this safety position can also be used as a safety measure when a maximum temperature is exceeded, in the case of a fire for example, when the pneumatic lines for the drive unit are made from a plastic that cannot withstand heat.
In practice, such drive units fail when, for example, a container or boiler must be opened in case of fire in order to prevent an explosion. If the pneumatic system fails, then it is necessary to keep the container closed to avoid environmental impact.
Therefore, there is a need for a pneumatic drive unit by which the two different safety positions can be achieved when the pneumatic system fails and when a maximum temperature is exceeded.